design guide 2

8/7/2019 design guide 2

1/49

ISO NEW ENGLAND PLANNING

PROCEDURE NO. 9 APPENDIX B

Transmission Owners and ISO-NE

Substation Bus Arrangement

Guideline Working Group Report

Presented to the NEPOOL Reliability Committee April 4, 2006

February 23, 2006 Revis ion 18


2/49

Contents

Page

INTRODUCTION 2

EXECUTIVE SUMMARY 2

RECOMMENDATIONS 3

RELATIVE BUS ARRANGEMENT COST

COMPARISON 6

FIGURE A 7

APPENDIX 8

SUBSTATION DESIGNS CONSIDERED 9 - 21

FIGURES 1-17 22 - 39

New England Standard Substation Bus Arrangement WorkingGroup

Company Working Group Member

Northeast Uti l i t ies Roger Zaklukiewicz - Chair

Central Maine Power William Sawyer

Central Maine Power Mike Smallwood

ISO-NE David Forrest

National Grid Dean Latulipe

Northeast Uti l i t ies George Becker

NSTAR Swapan Dey

United I l luminat ing Robert Pellegrini

VELCO John Fiske

VELCO Ed McGann


3/49

INTRODUCTION

At the June 10, 2005 Transmission Owners and ISO-NE meeting, a Working Group

was charged to review possible transmission system substation1 arrangements and

develop a substation bus arrangement guideline for New England. The Working

Groups charge was to review all possible transmission grid substation bus

arrangements and develop a guideline for substation arrangements that best meets

the current and future needs of New Englands Transmission Grid.

EXECUTIVE SUMMARY

The Working Groups scope definition statement to develop a standard substation

arrangement for bulk power system substations is:

The guideline for substation arrangements must support and promote the rel iabi l i ty

of the New England Transmission System. It must incorporate a basic design that is

readily expandable and simplifies substation switching to safely isolate facilities andequipment with minimum adverse impact on power flows. It must provide operating

flexibility, allow for efficient and effective maintenance of substation equipment,

provide for a safe working environment, be cost effective and have the support of the

New England Transmission Owners and ISO-NE.

The Working Group recommends that a breaker-and-a-half bus arrangement be

adopted as the ultimate design for substations that might become major substations

operating at 115 kV or higher. The Working Group further recommends that this

design be considered good utility practice in the ISO-NE approval processes for new

substations and major modifications to existing substations . However, the Working

Group recognizes that in specific cases a different design may also represent good

utility practice and thus merit ISO-NE approval as reliable and cost effective.

WORKING GROUP METHODOLOGY

The methodology used to review and evaluate all potential substation arrangements

and to select a design was as follows:

1. The design must provide for worker safety. The design shall promote

switching sequence consistency and eliminate any possible confusion during

switching operations. It must also promote ease of incorporating appropriate

working clearances into the design.

2. The design must promote the rel iabil i ty of the New England Transmission

Grid and be supported by the New England Transmission Owners and ISO-

NE. The design shall be such that the failure of any switching device in

conjunction with a first contingency protective relay operation or a scheduled

1 The term substat ion is sued throughout the report ; however, the term swi tching stat ion or stat ion may be

in terchanged i f the fac i l i t y te rminates on ly t ransmiss ion l i nes. A substa t ion must have an e lement which

transforms vol tages (t ransformer or autotransformers). A swi tching stat ion or stat ion interconnects t ransmission

lines only.


4/49

outage of any one piece of equipment will not result in the interruption of a

critical transmission path.

3. The design must provide operating flexibility and allow for any substation

element or piece of electrical equipment to be safely isolated with minimal

interruption or risk to the flow of power on the transmission grid.

4. The design must allow for future expandability based on site limitations and

land availability. The design shall promote consideration for expansion as

part of the initial design scope and equipment installation.

5. The design must promote the ef f ic ient and ef fect ive maintenance of

substation electrical equipment. The design shall promote familiarity with a

consistent physical design phi losophy and equ ipment arrangement

throughout New England.

6. The design must be cost

effective.

RECOMMENDATIONS

The following recommendations are the result of the Working Groups analysis of

many possible substation arrangements and their advantages/disadvantages with

respect to the methodology outlined above.

General

1. If these guidelines are accepted, then, this Working Group shall draft a

formal Planning Procedure to document their application, and submit it tothe NEPOOL Reliability Committee.

2. The Working Group recommends that this guideline be considered good

utility practice in the ISO-NE approval processes for new substations 115 kV

and above and major modifications to existing substations 115 kV and above.

3. The Working Group recommends that ISO-NE recognize that in specific cases

a different design may also represent g ood uti l i ty practice and thus may

merit approval as reliable and cost effective. These specific cases should be

dealt with in the ISO-NE approval process.

4. The Working Group recommends, where pract ical , that land shal l be

purchased and equipment shall be arranged to al low for a substation

breaker-and-a-half bus configuration when transmission grid expansions

dictate. It is not recommended that in all cases a substation initially be built

with a breaker-and-a-half bus arrangement, only tha t the design of the

substation provides for expansion to a breaker-and-a-half bus arrangement

when the substation might become a major substation.

5. The Working Group recommends the use of air-insulated bus; however, the

use of gas-insulated bus should be considered good utility practice when it is


5/49

required to provide for a substation arrangement that conforms to the

guidelines.

6. The Working Group recommends that, when designing a new substation or

doing a major modification to an existing substation, that the Transmission

Owner consider the potential for the future termination of transmission

elements. For example, if there is vacant space on existing transmission

rights-of-way emanating from the substation or the substation is located in

an area that is or will be deficient in generation, it may be prudent to plan

the substation for additional transmission elements. A discussion of the

ultimate number of transmission elements should be included in the

application (I.3.9) to ISO-NE.

345 kV Substations

1. For 345-kV substations, the Working Group recommends that the major

substation bus arrangement guideline be a breaker-and-a-half bus

arrangement with space provisions for a series tie breaker in each bay. Thisrequirement only applies to major substations, those that have a reasonable

future potential for a total of (4)2 or more transmission lines, 345-kV

autotransformers, and/or Generator-Step-Up transformers. (see Appendix -

Figure A)

2. Where the failure of a tie circuit breaker produces unacceptable operational

consequences, a series t ie breaker posit ion with no switches between

breakers, may be included in the substation design.

3. If a series tie breaker is installed, there is no intention of terminating a

transmission element between the two series tie breakers. Doing so would

resul t in a Breaker-and-a-third arrangement, which is not the

recommended substation bus arrangement. Therefore, the space provision

for the series tie breaker should leave sufficient space for the series breaker

only, and not for termination of an additional transmission element. No

isolating disconnect switches need to be installed between the two series

breakers.

4. The following transmission elements should terminate in a designated bay

position:

a . A t r ansmiss i on l i ne

b. An autotransformer interconnecting the 345kV transmission grid tothe 230-kV or 115kV transmission grids3

2 The elements in the 345-kV grid tend to be extremely crit ical to the transmission grid rel iabil i ty, thus the added

rel iabi l i ty of a breaker-and-a-hal f scheme wi l l be considered good ut i l i ty pract ice i f 4 or more t ransmission

connections are forecasted.

3 The reason 345-kv autotransfomers may not be terminated on the main busses is because of the design problem

imposed by breaker fai lure cont ingencies for breaker-and-a-hal f stat ions containing 3 or more autotransformers.

With 2 autotransformers at a stat ion (1 on each main bus), instal lat ion of the 3r d

is problematic because wherever

the3 r d

autotransformer is terminated, a breaker fai lure contingency could result in a tripping of 2 autotransformers.

Terminat ing the autotransformer in individual bays el iminates this potent ial operat ing problem.


6/49

c. A Large Generator interconnection.

5. Capacitor banks, reactors and load serving transformers (e.g. 345/34.5 kV)

may be connected to the main buses in the breaker-and-a-half bus

arrangement but must have their own 345-kV circuit breaker and their own

protective relaying that trips this circuit breaker.

230 and 115 kV Substations

6. For 230 and 115 kV substations, the Working Group recommends that the

guideline also be a breaker-and-a-half bus arrangement , but with no space

provisions for a series tie b reaker. This breaker-and-a-half requirement

only applies to major substations, those that have a reasonable future

potential for a total of (4) or more transmission lines, 230 kV

autotransformers, and/or Generator-Step-Up transformers.

7. The following transmission elements should terminate in a designated bay

position:

a. A network (non-radial ) t ransmission l ine4

b. An autotransformer interconnecting the 230kV transmission grid to

the 115kV transmission grid

c . A Large Generator i n terconnect ion

Note that 115/69-kV autotransformers may be connected to the 115-kV main

bus.

8. Capacitor banks and reactors may be connected to the main buses in the

Breaker-and-a-half bus arrangement but must have their own circuit

breaker and their own protective relaying that trips this circuit breaker.

9. Load serving transformers (e.g. 115/13.8 kV) may be connected to the main

buses in the Breaker-and-a-half bus a rrangement. These transformers may

be connected to the main bus util izing a circuit breaker, circuit switcher, or

disconnect switch.

69 kV Substations

The working group makes no recommendations for 69-kV substation designs.

4 The intent of the non-radial qualif ier is to al low radial l ines supplying load serving transformers (e.g., 115/13.8

kV) to be connected directly on the main bus.


7/49

RELATIVE BUS ARRANGEMENT COST COMPARISON

The selection of the appropriate station bus arrangement should meet a ll known or

anticipated transmission reliability, operating, and maintenance criteria and future

expansion requirements taking into account recognized acceptable contingencies and

the associated risks to the Transmission Grid.

To assist in the overall evaluation, Table 1 provides a high level cost comparison of

the various substation bus arrangements, should it be necessary to conduct an

economic comparison.

Table 1: Station Bus Arrangement Cost Comparison 5

Bus Arrangement

Approximate RelativeCost Comparison

Single Bus 100%

Sectionalized Bus 122%Main and Transfer Bus 176%

Ring Bus 156%

Breaker-and-a-half 158%

Double Bus - Double Breaker 214%

The comparison is based on the cost to install a four-circuit low-profile outdoor bus

arrangement with power circuit breakers in al l circuits. Power transformer and

land costs are not included in the cost comparison. In schemes uti l izing other

protective devices or different circuit quantities, the relative costs may vary from

those listed.

5 McDonald, John D. Electric Power Substations Engineering CRC Press, New York p. 3-6, 2003


8/49

FIGURE A


9/49

APPENDIX


10/49

SUBSTATION DESIGNS CONSIDERED

TRANSMISSION SUBSTATIONS

In this report, a substation is designated a Transmission Substation when it isdesigned to operate at 115 kV or higher,. Since one transmission substation may

supply several distribution substations and large loads, rel iabi l ity of service and

flexibility of operation are extremely important. The substation design must permit

faci l i t ies and electr ical equipment to be maintained without having to interrupt

other transmission l ines, transformers, or equipment necessary to maintain the

reliability and integrity of the transmission grid. The e mployment of multiple buses,

circuit breakers, and isolating disconnect switches for switching and isolation

provide the required system flexibi l ity. A transmission substation is distinguished

from a transmission switching station in that it incorporates a transformer(s) or

autotransformer(s) which convert the transmission vol tage to ei ther a lower

transmission voltage or a distribution voltage.

TRANSMISSION SWITCHING STATIONS

Transmission switching stations do not change system voltage from one level to

another and therefore do not contain power transformers. Depending on system

voltage, the equipment types and characteristics used in transmission switching

stations are identical to those used in transmission substations. The charge to the

Working Group did not include the detai l engineering design of the substation

faci l i t ies, electrical equipment, control houses, or i ts protection and controls

capabilities and circuitries. As future ISO-NE/Transmission Owner standards are

developed, consideration should be given to the development of such standards to

maximize the reliability of the New England transmission grid, minimize operating

complexities, maximize flexibility, reduce operating risks, and ensure worker safety.

UNIVERSAL SUBSTATION BUS ARRANGEMENTS

The Working Group conducted a paper search and studied each of the bus

arrangements most typically employed for transmission voltages th rough 765 kV in

North America and Western Europe. The Working Group recognized that initially, a

transmission substation or switching station might only terminate two transmission

l ines or terminate into a single transformer/autotransformer. However as the

transmission grid expands to accommodate growing load and additional generation,

that simple substation/switching station could develop into a ful l f ledged faci l i tyhaving signif icantly greater importance and requiring far greater f lexibi l i ty and

complexity. Therefore, the recommendations of the Working Group must

incorporate a stepped approach for those substations/switching stations that initially

are small which permits their expansion in a systematic manner without adversely

impacting the reliability of the transmission grid.

The physical size, type, and arrangement of major equipment, such as power

transformers, circuit breakers, disconnect switches, dimensions of the property

owned by the Transmission Owner, and location of adjacent transmission line rights


11/49

of way and private developments may cause variance in the layouts to suit

individual requirements. However, the construction of the new or expansion of the

existing bus arrangement should not compromise the preferred bus arrangement

guideline. The Working Groups scope includes recommending a preferred

substation/switching station bus arrangement; this does not include detailed

engineering/design of the facility.

SINGLE BUS ARRANGEMENT

A single substation bus arrangement consists of one main bus that is energized at

all times and to which all transmission lines and transformers are connected. Each

transmission line or transformer is electrically connected to the main bus through a

single circuit breaker. This arrangement is the simplest to design and operate, has

the least amount of equipment, but provides the least amount of f lexibi l i ty and

system rel iabi l i ty. Figure 1 depicts a possible single bus station arrangement. It is

not possible to perform maintenance work on the main bus or sections of a

transmission l ine or transformer directly connected to the bus without taking the

entire station out of service. Once in service, the entire station must be de-energizedto extend the main bus or to connect an additional transmission line or transformer

to the main bus.

Any bus fault or failure of a circuit breaker to operate under fault conditions will

result in complete loss of the station. The loss of the entire transmission station

could result in transmission line and equipment overloads, unusual power flows

throughout the transmission grid. Therefore, in both the planning and real t ime

contingency analysis studies, the loss of a single bus station must be a recognized

contingency when the studies are being performed.

On the distribution system, a single bus arrangement is not recommended without

providing circuit breaker bypass switching capability that permits circuit breaker

maintenance while maintaining circuit operation. Switches are provided that, when

closed, parallel two lines to enable one circuit breaker to be removed from service.

The other breaker then protects both circuits. Figure 2 i l lustrates a single bus

station with circuit breaker bypass switches. This arrangement, however, results in

the possible loss of overcurrent or step distance relay protection coordination for the

distribution circuit. A fault occurring on the l ine with the breaker bypassed could

result in the loss of multiple circuits or complete deenergization of the station. Such

operating risks are not acceptable on the transmission grid; therefore, a single bus

arrangement that employs bypass switch facilities should not be installed on the

transmission grid.

Advantages:

1 .Lowes t cos t2 . S m a l l l a n d a r e a r e q u i r e d

3.S imple const ruc t ion

4.S imple in concept and operat ion

5.Relatively simple for the application of protective relaying

Disadvantages:

11


12/49

1. A s ingle bus ar rangement has the lowest re l iabi l i ty

2. Has the least maintenance and operat ing f lex ib i l i ty

3. Circuit breaker maintenance requires opening the transmission line terminal or

removing the transformer from service

4. Highest potential for the loss of the entire station for a bus fault, a fault on any

element directly connected to the main bus or for the failure of a circuit breaker

5. Must remove the entire main bus from service to extend the bus or connect an

additional transmission line or transformer position

6. Circuit breaker bypass facilities do not provide for circuit protection when bypass

facilities are being used

Use of the circuit breaker bypass facilities to support maintenance can compromise

or disable some of the protective relay applications and overall relay coordination

Conclusion:

A single bus arrangement is not a suitable design for a major

transmission station.

SECTIONALIZED BUS ARRANGEMENT

An extension of the single bus configuration is the single bus station with bus

sectionalizing circuit breaker arrangement shown in Figures 3. This arrangement

electrical ly connects two or more single bus schemes with one or more bus

sect ional izing ci rcui t breakers. The sect ional izing ci rcui t breaker(s) may be

operated normally open or closed, depending on system requirements. In this

arrangement, a bus fault or breaker-failure causes only the affected bus section to be

removed from service and thus eliminates the risk of the loss of an entire station.Power flows on the transmission grid under normal and emergency conditions must

be taken into consideration when locating transmission l ines and transformers on

each section of the bus such that load is not separated from its source when the

sectionalizing circuit breaker is opened. This can be accomplished by positioning

interrelated transmission l ines on different bus sections to minimize or el iminate

entirely the possibility of abnormally low voltage or overloaded transmission line

conditions.

A main bus fault or breaker-fai lure condit ion can be isolated to the faulted bus

section without interrupting the entire station. The adverse impact of a bus fault or

breaker-failure on the transmission grid is appreciably less when a sectionalizing

circuit breaker is installed between the buses due to the increased flexibility of this

bus arrangement.

Advantages:

1.Greater operat ing f lexibi l i ty than a single bus arrangement

2.Higher re l iabi l i ty than a s ingle bus ar rangement

3.Abi l i ty to isolate bus sect ions for maintenance

12


13/49

4.S imple const ruc t ion

5.S imple in concept and operat ion

6.Loss of only part of the station fol lowing a bus fault or breaker-fai lure condit ion

7.Relat ively simple for the appl icat ion of protect ive relaying

Disadvantages:

1. A sectionalized bus arrangement has a higher cost than a single bus

arrangement

2. Addi t ional ci rcui t breakers are required for sect ional izing

3. Sectionalizing may cause isolation of source circuits from load circuits for a bus

fault, a fault on any element directly connected to a bus ,or for the failure of a

circuit breaker.

4. Circuit breaker maintenance requires opening the transmission line terminal or

removing the transformer from service

5. Must remove a bus section from service to extend the bus or connect an

additional transmission line or transformer position to that bus section

6. Potential for the loss of the entire station for a bus fault with the failure of thebus tie circuit breaker

Conclusion:

A sectionalize bus arrangement is not a suitable design for a

major transmission station.

MAIN AND TRANSFER BUS ARRANGEMENT

A main and transfer bus arrangement consists of two independent buses, one of

which, the main bus, is normal ly energized. During normal operat ions, al l

transmission l ines and transformers are electrical ly connected to the main bus

similar to that of a single bus station arrangement. When it is necessary to remove a

circuit breaker from service for maintenance or repair, it is possible to switch the

transmission l ine or transformer to the transfer bus such that there is no loss of

service. The isolation switches associated with the bus tie circuit breaker are closed,

the bus t ie circuit breaker is closed energizing the transfer bus, then the bypass

switch for the circuit breaker to be isolated is closed parallel ing the transmission

line or transformer to the main and transfer buses. The bypassed circuit breaker

and its isolation switches are then opened to remove the circuit breaker from

service. The transmission l ine or transformer is then protected by the protective

relaying package associated with the bus tie circuit breaker.

Figure 4 i l lustrates simplif ied main and transfer bus station arrangements. As with

the single bus arrangement it is possible to incorporate one or more sectionalizing

circuit breakers to provide additional operating flexibility and reliability. Figure 5

i l lustrates a main and transfer bus arrangement stat ion incorpo rat ing a

sectionalizing circuit breaker which minimizes the number of transmission lines and

transformers on each bus section. With this arrangement, there is appreciably less

13


14/49

risk that a bus fault or breaker-failure condition would result in the loss of the

entire station. In addition, its possible to incorporate more tailored protection

packages for each bus tie circuit breaker.

Advantages:

1. Ability to perform circuit breaker maintenance while maintaining transmission

line and transformer service

2. Reasonable in cost3 . F a i r l y s m a l l l a n d a re a

4. Where the transmission lines and transformers can be placed on the transfer bus

the main bus can be readily expanded without service interruption

Disadvantages:

1. An addi t ional ci rcui t breaker is required for the bus t ie posi t ion

2. Since the bus tie breaker has to be able to be substituted for any transmissionline or transformer circuit breaker, its associated protective relaying must have

the functional capability of the protective relaying for each element connected to

the bus and relay settings must be changed for each use of the transfer circuit

breaker.

3. Failure of a circuit breaker or a bus fault causes loss of the entire substation4. Somewhat complicated switching is required to remove a circuit breaker from

service for maintenance

5. May have to remove some transmission lines or transformers from service to

extend the bus o r connect an additional transmission line or transformer position

to that bus section

6. Loss of the entire station for a bus fault, a fault on any element directly

connected to the main bus or for the failure of a circuit breaker

Conclusion:

A main and transfer bus arrangement is not a suitable design for a


RING - BUS ARRANGEMENT

The ring-bus station arrangement is one o f the simplest and most commonly

constructed transmission bus arrangements in existence. In a r ing bus

arrangement, circuit breakers are placed between transmission lines or transformer

position as illustrated in Figure 6; there are always two circuit breakers associated

with each transmission l ine or transformer posit ion. Since each circuit breaker in

the ring is shared, it is possible to perform maintenance on each circuit breaker

without impacting the operation of the transmission line or transformer position on

either side of the circuit breaker.

14


15/49

Bus differential protection packages are not required because there is no designated

main or transfer bus in this arrangement. The section of the ring between circuit

breakers is always protected by the protective relaying package of each transmission

line or transformer position. The philosophy and application of the protection and

controls ci rcui t r ies for ei ther the transmission l ine or t ransformer posi t ion are

similar lending them to an ease of understanding of how each protective relay

package works. The required switching to safely isolate each transmission line or

transformer position is also similar which simplifies operations and should result in

improved system reliability.

Ring buses should be limited in total to four to six transmission line and transformer

positions in order to maximize the flexibility and reliability of this bus arrangement.

This l imi tat ion is dr iven by the two factors. Whenever maintenance is being

performed on a circuit breaker or one of its two isolating disconnect switches one

must open the bus which interrupts the normal flow of power around the ring bus.

In certain operating circumstances, power flows in sections of the bus could exceed

the current ratings of circuit breakers, disconnect switches, or other hardware.

More importantly, should a fault occur on a transmission l ine or transformerconnected to the bus, it is possible that source transmission lines could be isolated

from transmission l ines and transformer posi t ions which are serving load.

Whenever the ring bus must be opened to perform maintenance or during shorter

periods to perform switching, the reliability of the ring bus arrangement decreases

to that of the single bus arrangement with sectionalizing circuit breakers.

When system studies indicate that system rel iabi l i ty degrade s signif icantly when

two transmission l ines or one transmission l ine and one transformer are

simultaneously removed from service they should not share a common circuit

breaker. The most common case occurs whenever two transmission lines are located

on a common transmission structure, determining where be st to connect each

transmission line to the ring bus. If the transmission lines are connected in adjacent

posit ions both transmission l ines wil l be lost for a breaker-fai lure condit ion. If they

do not share adjacent positions in the ring, there is the possibility that the bus will

be split whenever a d ouble circuit transmission line fault occurs. The p robability of

each contingency occurring must be studied and incorporated in the decision of

where to locate each transmission line or transformer on the ring bus.

Once constructed and placed in service, it is difficult to expand the ring bus unless

planned for in the engineering/design phase. Of concern in New England are the

multiple transmission element outages that can occur following a breaker-failure

contingency. If i t is unacceptable to have adjacent posit ions of a ring bus out of

service simultaneously, a possible solution is to install two circuit breakers in seriesas i l lustrated in Figure 7. In this conf igurat ion a breaker-fai lure of the ci rcui t

breakers associated with either Line 1 or Line 2 nearest transformer T1, would not

result in the simultaneous loss of transformer T1. It is not possible to predict how

the transmission gr id wi l l develop; therefore, i f r ing-bus stat ions a re ini t ial ly

instal led it may be prudent to al low for the addit ion of a series circuit breaker on

either side of each circuit breaker. A second alternative would be to design the bus

so that i t can be easily expandable to a breaker-and-a-half arrangement as

i l lustrated in Figure 8.

15


16/49

Advantages:

1 . F l ex i b le opera t ion

2. Ease of understanding zones of protection, controls, and personal safety

3 . H i g h r e l i a b i l i t y

4. Isolation of circuit breakers for maintenance without disrupting transmission

line or transformer operation

5. Double feed to each transmission l ine or t ransformer posi t ion

6 . N o m a i n b u s es

7. Expandable to breaker-and-a-hal f bus ar rangement

8. Lowest cost bus design wi th s imi lar re liabi l i ty and flex ib i l ity

Disadvantages:

1. Ring bus wil l spl i t for faul ts on two transmission l ines on a common

transmission structure or for a fault on a transmission line or transformer

position during breaker maintenance to leave possibly undesirable circuitcombinations (supply/load) on the remaining bus sections.

2. Each circui t has to have i ts own potential source for protect ive relaying.

3. This configuration is usually limited to four circuit positions, although larger

rings are in service. A 6-postion ring bus is usually considered as a maximum

limit for the number of terminals in a ring bus.

4. The protective relaying and controls schemes are more involved since each

circuit breaker has to respond to faults on two circuits.

5. Automatic reclose schemes may be more complex since each circuit breaker may

have different control modes depending upon which transmission l ine or

transformer position trips.

6. Di f f icul t to expand unless ini t ial design incorporates that f lexibi l i ty

Conclusion:

A ring bus arrangement is not a suitable design for a major

transmission station. However a ring bus that can be expanded into a

breaker-and-a-half bus arrangement would be suitable for the

initia l desig n of a substat ion that may become a major substation in

the future.

BREAKER - AND - A - HALF BUS ARRANGEMENT

The breaker-and-a-half bus arrangement is relatively simple and consists of two

main buses, each normally energized. Between each of the main buses are similarly

arranged bays of three circuit breakers configured such that the two transmission

lines or combination transmission l ine and transformer posit ion share the center

circuit breaker as i l lustrated in Figure 9. Each transmission l ine o r transformer

position shares the common center circuit breaker, so there are one-and-a-half

16


17/49

circuit breakers per position; therefore, the designation a breaker-and-a-half bus

arrangement.

The breaker-and-a-half bus arrangement is the most prominently used transmission

substation configuration in North America. It is more rel iable than a ring-bus

arrangement and provides appreciably more operating flexibility.

Similar to a ring-bus configuration, each transmission l ine or transformer posit ion

can be served from two directions. It is possible to maintain one of the two circuit

breakers associated with each transmission l ine or transformer posit ion without

impacting power flow. Since each of the bays connects to a main bus, the breaker-

and-a-half bus arrangement does not have the operating restrictions of a ring bus

arrangement when work is being performed on a circuit breaker as the main buses

are designed to carry appreciably greater current flows than the c ircuit breakers and

buses in any given bay. Simi lar to a r ing-bus conf igurat ion, one should not

electrically connect two transmission lines or a transmission line and a transformer

that cannot be simultaneous out of service to adjacent positions in a given bay. A

breaker-failure of the shared circuit breaker would require the interruption of powerflow to both positions should such a contingency occur. Additionally, faults on either

of the main buses cause no circuit interruptions

The breaker-and-a-half bus arrangement has little advantage over the ring bus

arrangement when there are in total fewer than four t ransmission l ines or

transformer station positions. Breaker-and-a-half stations are more expensive to

construct than ring bus stations; therefore, if the number of positions is to remain

limited well into the future, designing and constructing a ring bus station would be

the more cost effective solution. The breaker-and-a-half arrangement should be

used for stations where there a large number of transmission lines and transformer

positions terminating at that location.

At those locat ions where the number of t ransmission r ight of way corr idors is

l imited and a majority of the transmission l ines must exit on a single transmission

right of way a variation of the basic breaker-and-a-half station may be preferred.

Figure 10, il lustrates such an arrangement. The folded breaker-and-a-half station

arrangement is electrically identical to the bus arrangement in Figure 9; however, it

is designed to assist transmission l ine engineers maximize the number of

transmission lines that can exit the station where limited right of ways exist.

As with ring-bus stations, once a breaker-and-a-half station is constructed and

placed in service, i t is diff icult to place a second breaker in series with a circuit

breaker unless provisions are incorporated during the initial engineering design ofthe station. Transmission grid operating conditions may n ot allow the simultaneous

loss of two transmission l ines or the combination of a transmission l ine and a

transformer posit ion fol lowing a breaker-fai lure. In a breaker-and-a-half station

arrangement, such design provisions only need to be incorporated for the center or

shared circuit breaker position. If one of the circuit breakers directly connected to

ei ther main bus posi t ion were to fai l , only the transmission l ine or t ransformer

posi t ion direct ly connected to the posi t ion would remain impacted. Figure 11

illustrates possible connections of two circuit breakers in the center or shared circuit

breaker position.

17


18/49

Figures 12 and 12A illustrate an autotransformer electrically connected to one of the

main buses through a disconnect switch. Should there be a faul t in the

autotransformer, it would be necessary to trip all circuit breakers directly connected

to that main bus and the transformers low voltage bus circuit breaker(s) to isolate

the fault from the transmission grid. The ISO-NE standard should not al low a

Transmission Owner to electrical ly connect any circuit element to the main bus

without a circuit breaker or a circuit switcher. The reliability of the entire station

decreases when direct connection of a capacitor bank, a transformer or a

transmission l ine to the main bus is permit ted. Operat ing f lexibi l i ty is also

negatively impacted requiring the main bus to remain out of service anytime work

must be performed on the disconnect switch or circuit switcher. The cost to install

the addit ional circuit breaker is not tr ivial; however, the potential negative impacts

to the breaker-and-a-half station outweigh those added costs. It is the working

groups recommendation that such electrical connections not be permitted.

Advantages:

1. Greater operat ing f l ex ibi l i t y than a r i ng bus

2 . H i gher re l i ab i li t y t han a r i ng bus

3. Abi l i ty to isolate ei ther main bus for maintenance without disrupt ing service

4. Can isolate any circuit breaker for maintenance without disrupting service to the

transmission l ine or transformer

5. Double feed to each transmission l ine or t ransformer6. Bus faul t does not interrupt service to any transmission l ine or t ransformer

7 . A l l sw itch ing per formed w i th c i rcu i t b reakers

8. Maintenance of circuit breaker or isolating disconnect switch does not open the

bus should a fault occur during the maintenance period

9. Appreciably less r isk of isolat ing Transmission gr id sources from the load

10. Easy to add a double bus doub le breaker bay

Disadvantages:

1. Greater costs than a r ing bus stat ion ar rangement

2. One-and-a-hal f c ircui t breakers are required per c i rcuit

3. Utilization of different protective relaying and control philosophies within the

station

4. The protective relaying and control schemes are more involved and complex since

the center or common circuit breaker has to respond to faults on two circuits

5. Automatic reclose schemes associated with the center or common circuit

breakers may be more complex since each circuit breaker may have different

control modes depending upon which transmission line or transformer position

trips

6. Each transmission l ine requires i ts own potent ial source for relaying

7. Must remove one of the bus sections from service to extend the bus or electrically

connect another bay to the station

8. Difficult to add a second circuit breaker in series with the center or common

circuit breaker unless space is provided in the original station design

18


19/49

9. A bus fault requires the tripping of one breaker per bay, therefore increasing the

potential for a breaker-failure contingency to occur if there are many bays

Conclusion:

A breaker-and-a half bus arrangement is a suitable design for a


BREAKER - AND - A - THIRD BUS ARRANGEMENT

The breaker-and-a-third uti l izes four circuit breakers between its main buses and

has two common circuit breakers. The reliability of a breaker and third bay is lower

than the bay of a breaker-and-a-half arrangement; however, i t is better than that of

a ring bus station. A breaker-fai lure of either of the shared circuit breakers wil l

result in the loss of two transmission facilities (line or transformer) similar to that in

a breaker-and-a-half station. The difference being that with a breaker-and-a-thirdbus arrangement the probability that this contingency might occur is twice as great.

Figure 13 il lustrates a breaker-and-a-third bus arrangement.

Advantages:

1. Greater operat ing f l ex ibi l i t y than a r i ng bus

2 . H i gher re l i ab i li t y t han a r i ng bus


4. Can isolate any circuit breaker for maintenance without disrupting service to the

transmission l ine or transformer

5. Double feed to each transmission l ine or t ransformer

6. Bus faul t does not interrupt service to any transmission l ine or t ransformer7 . A l l sw itch ing per formed w i th c i rcu i t b reakers



9. Appreciably less r isk of isolat ing Transmission gr id sources from the load

Disadvantages:

1. Greater costs than a r ing bus stat ion ar rangement

2. One-and-a- th i rd c i rcui t breakers are required per c i rcui t


station

4. The protective relaying and control schemes are more involved and complex since

the two center or common circuit breakers h ave to respond to faults on two

circuits

5. Automatic reclosing schemes associated with the two center or common circuit

breakers may be more complex since each circuit breaker may have different

control modes depending upon which transmission line or transformer position

trips

6. Each transmission l ine requires i ts own potent ial source for relaying

19


20/49

7. Must remove one of the bus sections from service to extend the bus or electrically

connect another bay to the station

8. Difficult to add a second circuit breaker in series with either of the center or common

circuit breakers unless space is provided in the o riginal station design

9. The risk of a failure of a shared breaker may be twice as great as that for that of a

breaker-and-a-half bus arrangement.

10. A bus fault requires the tripping of one breaker per bay, therefore increasing the

potential for a breaker-failure contingency to occur if there are many bays

Conclusion:

A breaker-and-a -third bus arrangement is a suitable design for a major

transmission station. In cases where a breaker-and-a-half arrangement

is not feasible, either physical ly, environmental ly, or economical ly,

a breaker-and-a-third bus arrangement may be an acceptable

alternative.

DOUBLE BUS

DOUBLE BREAKER ARRANGEMENT

The double bus - double breaker bus arrangement is employed where ult imate stat ion

rel iabi l i ty is required. This bus arrangement provides the greatest rel iabi l i ty and

flexibi l i ty and has the greates t co st. Th e two main buses are normally energized; in

each designated bay between the main buses are two circuit breakers and, between the

circuit breakers is a transmission line or transformer position as il lustrated in Figure 15.

In the double bus - double breaker arrangement, any circuit breaker can be removed from

service wi thout interrupt ing the power f lows on any transmission l ine, t ransformer

posi t ion or the main bus. A faul t on ei ther main bus has no direct impact on the

power f lowing o n any tran smission l ine or transformer posit ion. Equally important, a

breaker-failure contingency results in the loss of only one transmission line ortransformer position.

The protection and control schemes for each transmission line and transformer

position will be similar minimizing the complexity of the schemes associated with the

center or common b reaker of a b reaker-and-a-half station. The required switching

and tagging for each transmission line or transformer position will be similar, which

should make it easier for the workers to understand and switch.

Use of the double bus - double breaker has tradit ionally been l imited to large

generating stations because of its higher cost. The additional reliability afforded by this

arrangement over the breaker-and-a-half scheme usually cannot be justified for conventional

transmission stations. Occasionally, one bay of a breaker-and-a-half arrangement is used

as a double bus - double breaker arrangement for a generator lead, critica l tran smission

line, o r large autotran sformer, that re quires access to either main bus.

Advantages:

1. Greatest operation f lexibi l i ty

20


21/49

2 . V e r y h i g h re l i a b i l i t y


4. Can isolate any circuit breaker for maintenance without disrupting transmission

line or transformer operation

5. Double feed to each transmission l ine or t ransformer posi t ion

6. Bus fault does not interruption service to any transmission l ine or transformer

7. Loss of only one transmission line or transformer position following a breaker-

failure contingency

8. No need to place two circuit breakers in series to negate the negative impacts of

a breaker-failure contingency



10. A l l swi tching pe r formed wi th c i rcu i t breakers

11. Least r isk of isolat ing Transmission gr id sources from load

12. Similar protective relaying and control schemes for each bay

13. Switching for each transmission l ine or t ransformer posi t ion is simi lar

Disadvantages:

1. This bus conf igurat ion has the highest cost

2. Two circuit breakers are required for each transmission line or transformer

position


station

4. Each transmission line or transformer position requires its own potential source

for relaying

5. Must remove both of the main buses from service to extend the bus or electrically

connect another bus to the station

6. Requi res more land than a breaker-and-a-hal f stat ion

7. A bus fault requires the tripping of one circuit breaker per bay, therefore

increases the potential for a breaker-failure contingency to occur if there are

many bays

Conclusion:

Under normal c ircumstances, a double-bus-double-breaker bus

arrangement is too costly a design to be considered good utility

practice for a major transmission substation. However there may be

specific cases where the use of this bus arrangement may be

prudent, to address real estate constraints or the need for greater

reliability.

21


22/49

FIGURES 1-17

22


23/49


24/49

0-- LINE 1

LINE 2

i

FIGURE 2 : SINGLE BUSARRANGEMENT WITHBREAKER BYPASSSWITCHES


25/49

L I N E 4 - u L I N E 3

T2--, LINE 2

LINE 1

_LINE 4

_LINE 2

_ T1

_- LINE 1

FIGURE 3 :SINGLE BUS STATIONSWITH SECTIONALIZING

BREAKER


26/49

/

- _ -LINE 2E E___/ i1.--- LINE 1

MAINBUS

E E

E E

TRANSFER MAIN

BUS BUS

T,_

][ 1 ---

L_


27/49

TRANSFERBUS

FIGURE4 :MAIN&TRANSF

ERBUSARRANGEMENTS


28/49

L INE 4

1.-- LINE 3

'''''',....______.-

------____.

__'

'I'*s..............

_1.- LINE 2

T1

,--- LINE 1

MAIN TRANSFERBUS BUS

FIGURE 5 :MAIN & TRANSFERARRANGEMENT WITH BUSSECTIONALIZING BREAKERS


29/49

a

/1

0

/ LINE 4

0

/

E 2

E 3

.

FIGURE 6 :RING-BUS ARRANGEMENT


30/49

LINE 4 / LINE 5

/ LINE 1LINE 3

LINE2

1

A

I

II

Le

L e/),


31/49

)

1 _

_ _

_ _

E l

/.-

L

j [nT1

R


32/49

ING-BUSARR

ANGEMENTWI

THFIGURE7:


33/49

PROVISIONS

FORTWOBRE

AKERSINSER


34/49

IES.


35/49

LINE1 [ T 1

---0---L---0---1 \--0----.

LINE 2 LINE 3-... ...-

.-,-0------E1---L--0----.

L I N E 5 F LINE 41I II I\ i

---D-- --i---D--- --- .....-

FIGURE 8 : RING-BUS EXPANDABLE TO BREAKERAND-A-HALF BUSARRANGEMENT


36/49

LINE 51.--LINE 2

LINE T1

LINE 3 LINE

FIGURE 9 : BASIC BREAKER-AND-A-HALFBUS ARRANGEMENT


37/49

LINE 1 LINE 2 LINE 3 T1 LINE 5

F IGURE 10 :FOLDED BREAKER-AND-A-HALF BUS

ARRANGEMENT


38/49

LINE 3 LINE 4

LINE 1 T1

LINE 2 LINE 5

FIGURE 1 1 : BREAKER-AND-A-HALF BUS ARRANGEMENT

WITH PROVISIONS FOR TWO

BREAKERS IN SERIES


39/49

LINE 4

LINE 3

L

LOW VOLTAGEBUS

LOW VOLTAGEBUS

LINE 2 LINE 5

FIGURE 12A :BREAKER-AND-A-HALF BUS ARRANGEMENTWITH AUTO TRANSFORMER CONNECTED TO A MAIN

BUS


40/49

LINE 4

LINE 3

LINE 1 LINE 6

LINE 2 LINE

LOW VOLTAGE LOW VOLTAGEBUS BUS

FIGURE 12A :BREAKER-AND-A-HALF BUS ARRANGEMENTWITH AUTO TRANSFORMER CONNECTED TO A

MAIN BUS


41/49

LINE 4

LINE 7 LINE 8

LINE 1

LINE

2

__LINE

5

LINE 3

LINE 6


42/49

GURE1

3:COMBINA

TIONBREAKER-AND-A-

HALFANDB

R


43/49

EA

KER-AND-A-

THIRDBUSARRANGEME

NT


44/49

FIGURE 14 :DOUBLE BUS-DOUBLE BREAKER BUS ARRANGEMENT


45/49

LINE 4..-LINE 1

LINE 3.-INE 2

CRITICAL LINE or AUTOTRANSFORMER

-0--,---1--0---

i---E3--

- 1 = 1 - - - - - - - - E 1 - -

L - - E 1 -


46/49

FIGURE 15 :COMBINATIONDOUBLE BUSDOUBLE BREAKERAND BREAKERANDAHALF BUSARRANGEMENT


47/49

FIGURE 16 : CROSSED-RING-BUS ARRANGEMENT


48/49

LINE 1,LINE 8

LINE 2,LINE 7

LINE 3

LINE 4,

LIN

E 6

LINE 5

-or

FIGURE 17 : RING-

BUS


49/49

ARRANGEMENTWITHBRIDGINGCIRC

UITBREAKER

design guide 2

Documents